[BUG] CONTRIBUTE.md references deprecated 'poetry shell'

- [x] I've checked [docs](https://rich.readthedocs.io/en/latest/introduction.html) and [closed issues](https://github.com/Textualize/rich/issues?q=is%3Aissue+is%3Aclosed) for possible solutions.
- [x] I can't find my issue in the [FAQ](https://github.com/Textualize/rich/blob/master/FAQ.md).

**Describe the bug**

Since Poetry (2.0.0), the shell command is not installed by default.


